Method for determining channel quality indicator, base station and user equipment therefor

ABSTRACT

A base station and a related method are provided. The base station includes a transmitter which, in operation, transmits, to a user equipment (UE), values configured for different base station coordination types, respectively, wherein each of the base station coordination types defines which base stations among multiple base stations perform coordinated transmission in that base station coordination type. The base station further includes a receiver which, in operation, receives a channel quality indicator (CQI) reported from the UE, wherein the CQI is calculated at the UE in reference to the values.

TECHNICAL FIELD

The present disclosure relates to Channel Quality Indicator (CQI)determination and traffic offloading in base station cooperation.

BACKGROUND

Base station cooperation in a heterogeneous network is an importantmeans to improve the performance, e.g., throughput, of communicationsystems. In the heterogeneous network, the transmission power fromdifferent base stations may be very different and accordingly thecoverage areas of the base stations are different. Specifically, thecoverage area of a high power base station, i.e. a high power node(macro) is much larger than the coverage area of a low power basestation, i.e. a lower power node (LPN), and the coverage area of the LPNmay overlap with that of the macro, as shown in FIG. 1. Because of thelarge coverage area of the macro and the small coverage area of the LPN,a large number of UEs are in the coverage area of the macro and will beserved by the macro while only a small portion of UEs will be served bythe LPNs, which results in a heavy load on the macro and an inefficientuse of spectral resources of LPNs. To improve the spectral usage of LPNsas well as to reduce the load on the macro, it is preferred to assignmore UEs to be served by the LPN, even when the power received from themacro is higher than the power received from the LPN. This is a socalled traffic offloading.

There are multiple possible types of base station cooperation, and eachtype may yield a traffic offloading in different level. For example, asshown in FIG. 1, a joint transmission (JT) from the macro and the LPN1or LPN2 causes more macro traffic than a JT from the LPN1 and LPN2, andhence is less preferred from the traffic offloading point of view. Inother words, in case the macro is heavily loaded and the LPNs arelightly loaded, it is desired to avoid the joint transmission involvingdata transmitted from the macro, e.g., the joint transmission from macroand LPN1. In this document, joint transmission refers to that the UEreceives data from multiple base stations simultaneously.

A simple operation that enables traffic offloading may be as following:first, each UE report one CQI to base stations, where the CQI iscalculated at UE assuming a base station cooperation type friendly totraffic offloading; second, a scheduler included in the base station(which could alternatively be located separately from the base stationsin the heterogeneous network) calculates Proportional Fairness (PF)metric of each UE based on the reported CQI to make a resourceallocation; third, the scheduler allocates for example Physical DownlinkShared Channel (PDSCH) to UEs based on the calculated PF metric; notethat here the PDSCH transmission in general may follow the base stationcooperation assumed in UE CQI calculation.

To enable the more flexible traffic offloading, a further operation maybe as following: first, each UE report two CQIs to base stations, wheredifferent CQI is calculated assuming different types of base stationcooperation (and consequently assuming different traffic load); second,based on the load condition, a scheduler included in the base station(which could alternatively be located separately from the base stationsin the heterogeneous network) internally decide on which subframe(s) themacro is allowed to carry out the first type and the second type of basestation cooperation, respectively; third, the scheduler calculatesProportional Fairness (PF) metric of each UE on a certain subframe tomake a resource allocation, the PF metric is calculated based on allowedbase station cooperation on that certain subframe; fourth, the schedulerallocates for example PDSCH to UEs based on the calculated PF metric;note that here the PDSCH transmission in general may follow the basestation cooperation assumed in the relevant UE CQI calculation.

Both of the above operations require the UE to report CQI(s). Ingeneral, different types of base station cooperation lead to differentCQIs. Therefore, there is a problem how to decide which base stationcooperation should be assumed in CQI calculation. In short, the problemis how to decide which CQI should be reported.

The first possible solution is to let the UE decide which CQI(s) shouldbe reported. However the UE selected CQI could be very off because thetraffic load conditions at base stations are unknown to the UE. If theUE merely selects the highest CQI(s), then it is likely that the highestCQI(s) cause heavy traffic load at the macro, which means that trafficoffloading operation at base stations is not possible.

The second possible solution is to feedback CQIs based on a base stationsignalling. That is to say, the base station indicates UE which two CQIsshould be reported. However, this solution has following defects. On onehand, the base station selection is not optimal because the base stationdoes not know which CQI(s) are the highest at UE. Therefore base stationselection does not guarantee optimal performance. On the other hand,even if in some case the base station has knowledge of which CQI(s) arethe highest, the downlink signalling overhead is too much becausehighest CQI(s) may change dynamically. For example, if the signalling issend through Physical Downlink Control Channel (PDCCH), which is adynamic downlink signalling, the downlink signalling overhead may beintolerable. If the signalling is send through Radio Resource Control(RRC), which is a semi-static downlink signalling, the configuration cannot be changed quickly. Because the highest CQI may changes much fasterthan RRC configuration, the semi-statically configuration from basestation maybe not yields best performance.

SUMMARY OF THE DISCLOSURE

In one aspect of the present disclosure, there is provided a method fordetermining Channel Quality Indicators (CQI) to be reported by UserEquipments (UE), comprising steps of: assigning a sequence of adjustmentfactors to various possible base station cooperation; transmitting theassigned adjustment factors to UEs; calculating an original CQI for eachpossible base station cooperation in each UE; adjusting the originalCQIs based on the received adjustment factors in UEs; and reporting oneor multiple original CQI(s) that are the highest after the aforesaidadjustment and knowledge of which CQI is reported to the base station byeach UE.

In another aspect of the present disclosure, there is provided a methodfor dynamically determining Channel Quality Indicators (CQI) to bereported by User Terminals (UE), comprising the steps of: assigning anadjustment factor to each base station; transmitting the assignedadjustment factors to UEs; calculating an original CQI for each possiblebase station cooperations in each UE; adjusting the original CQIs basedon the received adjustment factors in UEs; and reporting one or multipleoriginal CQI(s) that are the highest after the aforesaid adjustment andknowledge of which CQI is reported to the base station by each UE.

In a further aspect of the present disclosure, there is provided a basestation for determining Channel Quality Indicators to be reported,comprising: an assignment unit, assigning adjustment factors used toadjust original CQIs; an adjustment factor transmitting unit,transmitting the assigned adjustment factors to User Equipments; areceiving unit, receiving the knowledge of which CQI is reported and thecorresponding CQI from User Equipments; a scheduling unit, whichallocates spectral resources to User Equipments based on the knowledgeof which CQI is reported and the corresponding CQI.

In a further aspect of the present disclosure, there is provided a userequipment for determining Channel Quality Indicators to be reported,comprising: an adjustment receiving unit, receiving adjustment factorsfrom a base station; a calculation unit, calculating an original CQI foreach possible base station cooperation; an adjustment unit, adjustingthe original CQIs based on the received adjustment factors; a reportingunit, reporting one or multiple original CQI(s) that are the highestafter the aforesaid adjustment and knowledge of which CQI is reported tothe base station.

In the present disclosure, UEs dynamically determine the optimal CQIs tobe reported by adjusting the original CQIs based on the adjustmentfactors assigned by the base station. Thus, the traffic offloading isenabled and the defect of performance loss can be fixed.

The foregoing is a summary and thus contains, by necessity,simplifications, generalization, and omissions of details; consequently,those skilled in the art will appreciate that the summary isillustrative only and is not intended to be in any way limiting. Otheraspects, features, and advantages of the devices and/or processes and/orother subject matters described herein will become apparent in theteachings set forth herein. The summary is provided to introduce aselection of concepts in a simplified form that are further describedbelow in the Detailed Description. This summary is not intended toidentify key features or essential features of the claimed subjectmatter, nor is it intended to be used as an aid in determining the scopeof the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features of the present disclosure will becomemore fully apparent from the following description and appended claims,taken in conjunction with the accompanying drawings. Understanding thatthese drawings depict only several embodiments in accordance with thedisclosure and are, therefore, not to be considered limiting of itsscope, the disclosure will be described with additional specificity anddetail through use of the accompanying drawings, in which:

FIG. 1 is a schematic diagram showing an example of a heterogeneousnetwork;

FIG. 2A is flowchart illustrating a method for dynamically determiningCQIs to be reported by UEs in base station cooperation according to thefirst embodiment of the present disclosure;

FIG. 2B is a table (Table 1) listing many possible types of base stationcooperation;

FIG. 3 is a diagram schematically illustrating a configuration exampleof the scheduler according to the present disclosure; and

FIG. 4 is a diagram schematically illustrating a configuration exampleof the UE according to the present disclosure.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof. In the drawings,similar symbols typically identify similar components, unless contextdictates otherwise. It will be readily understood that the aspects ofthe present disclosure can be arranged, substituted, combined, anddesigned in a wide variety of different configurations, all of which areexplicitly contemplated and make part of this disclosure.

First Embodiment

A method of dynamically determining Channel Quality Indicators (CQI) tobe reported by User Equipments (UE) is described in the presentembodiment. According to this method, the UE calculates original CQIsaccording to CQI calculation hypotheses corresponding to various basestation cooperations; adjusts the original CQIs based on adjustmentfactors indicated by the base station; and report the proper CQIs.Through this method, when the adjustment factors reflects the loadconditions at base stations, the UE can report CQIs by taking intoaccount the traffic load conditions as indicated by the base station.Therefore, the traffic offloading is enabled and the defect ofperformance loss can be avoided.

FIG. 2A is a diagram showing a flow chart of a method of dynamicallydetermining CQIs to be reported according to the first embodiment of thepresent disclosure. In the following, the method will be described indetail with reference to FIG. 2A, taking a macro and two LPNs (LPN1 andLPN2) carrying out the base station cooperation as an example.

As shown in table 1 in FIG. 2B, there are many possible types of basestation cooperation, and each of them has a traffic offloading indifferent level. In step 21, a base station assigns a CQI adjustmentfactor, for example, a CQI reduction value, for each possible basestation cooperation, where the sequence of CQI adjustment factor may bedecided based on the load conditions at multiple base stations or anyother factors, such as the backhaul condition of network. For example,if the macro is heavily loaded, while LPN1 and LPN2 are lightly loaded,the scheduler would configure base station cooperations that the macrotransmits data therein with high CQI reduction values. And the moreheavily the macro is loaded, the higher the CQI reduction value isassigned to the related base station cooperation. As a certain instance,the CQI reduction values may as shown in the column of “Macro HeavyLoad” in table 1. For another example, if the macro and the LPN1 areheavily loaded, the scheduler would configure base station cooperationsthat the macro or the LPN1 transmits data therein with high CQIreductions. As a certain instance, the CQI reduction values may as shownin the column of “Macro/LPN 1 HL” in table 1. Then, in step 22, theassigned CQI reduction values are transmitted to UEs. As an example, thereduction values may be transmitted through RRC or MAC (Media AccessControl) or PDCCH (Physical Downlink Control Channel).

In step 23, the UE calculates an original CQI for each possible basestation cooperation. Specifically, when the UE is requested to reportCQIs or prepares to report CQI periodically, UE calculates original CQIsbased on various possible CQI calculation hypotheses each corresponds toa possible base station cooperation. There are various methods ofcalculating original CQIs, a possible one could be the UE calculatessignal power, interference power and noise power respectively, and thenbased on reference signals and relevant data from base station(s), mapthe signal to interference and noise ratio (SINR) to certain modulationand coding scheme (MCS) according to a predefined CQI table. The detailsof how to calculate signal power, interference power and noise power aswell as how to map the SINR to certain modulation and coding based onreference signals and relevant data from base stations are well knownfor those skilled in the art and also out of the scope of thisdisclosure, which are henceforth omitted. As shown in table 1 in FIG.26, in case the base station cooperation is carried out by a macro andtwo LPNs (LPN1 and LPN2), there are 19 possible base stationcooperations and accordingly 19 CQI calculations hypotheses. So the UEcalculates original CQIs according to the 19 CQI calculation hypothesesand obtains 19 original CQIs. As a certain instance, the original CQIsmay as shown in the column of “Orig. CQI” in table 1. It is obvious thatthe original CQIs are calculated without considering the load conditionon the base stations.

Then, in step 24, the UE adjusts the original CQIs based on the receivedadjustment factors. In the present embodiment, the adjustment factorsare CQI reduction values. Therefore, with respect to each original CQI,the UE subtract the corresponding CQI reduction values therefrom, andthen obtains the adjusted CQI. For example, as shown in table 1 in FIG.2B, with respect to the case that the macro is heavily loaded and LPN1and LPN2 are lightly loaded, UE subtracts the values shown in the columnof “Macro Heavy Load” from the original CQIs shown in the column of“Orig. CQI”, and obtains the adjusted CQIs as shown in the column of“New CQI1”. For another example, with respect to the case that the macroand LPN1 are heavily loaded, UE subtracts the values shown in the columnof “Macro/LPN1 HL” from the original CQIs shown in the column of “Orig.CQI”, and gets the adjusted CQIs as shown in the column of “New CQI2”.

Then, in step 25, UE reports to the base station the original CQI thatare the highest after the aforesaid adjustment and the knowledge ofwhich CQI is reported. As shown in table 1 in FIG. 2B, if the CQIs arenot adjusted, the original CQI corresponding to the base stationcooperation shown in hypothesis 1 (i.e. Macro and two LPNs all transmitdata therein) will be reported. However, after applying the CQI adjustfactor, the original CQI corresponding to the base station cooperationshown in hypotheses 7 (i.e. two LPNs transmit data therein) is reportedin case that the Macro is heavily loaded, and the original CQIcorresponding to the base station cooperation shown in hypotheses 16(i.e. only LPN2 transmits data therein) is reported in case that theMacro and LPN1 are heavily loaded.

It can be seen, through the above described method according to thepresent embodiment, optimal CQI is reported considering the traffic loadcondition on the base station, and thus enables the traffic offloadingoperation on the base station.

On the other hand, if there is a base station particularly friendly tothe UE, for example, the original CQI calculated based on hypothesis 12results CQI=13. Then UE would still report CQI in hypothesis 12 afterapplying the aforesaid adjustment step of the method according to thepresent embodiment, although LPN1 is heavily loaded.

It should be noted that although in the above description UE reports thehighest CQI to the base station in step 25, however, it is only anexample and not a limitation. In fact, UE can report two or multiplehighest CQIs to the base station.

It should also be noted that although in the present embodiment themethod is described in an order from step 21 to step 25, however, thecarrying out order of the steps is not fixed. For example, step 23 canbe carried out before steps 21 and 22.

Furthermore, in the above description, CQI reduction values arepositive, so the original CQIs are reduced by subtracting the CQIreduction values therefrom. However, the CQI reduction values can alsobe negative. For example, when a base station is lightly loaded, anegative CQI reduction value, such as −5, can be assigned to the basestation cooperation which involves data transmission from thatparticular base station. In that case, the original CQI calculated basedon the CQI hypothesis corresponding to the base station cooperation willbe increased by subtracting the negative CQI reduction value therefrom.

Second Embodiment

The first embodiment is described above by taking the CQI reductionvalue as an example of the adjust factor. In fact, the adjust factor canbe any other types of value, such as weighting coefficient. For example,if the macro is heavily loaded, while LPN1 and LPN2 are lightly loaded,the base station cooperations that the macro transmits data therein maybe configured with a small weighting coefficient. And the more heavilythe macro is loaded, the smaller weighting coefficient may be assignedto the related base station cooperation. In this instance, in step 24,the UE adjusts the original CQIs by multiplying each original CQI by acorresponding weighting coefficient. The operations in other steps arethe same with those in the first embodiment, and thus will not bedescribed repeatedly herein.

Third Embodiment

The method of the present embodiment is substantially the same as thatof the first embodiment, except that the base station transmits to UEsmultiple sequences of adjustment factors. Each sequence may correspondto different load conditions. Specifically, in the first embodiment, asequence of adjustment factors is assigned to various possible basestation cooperations, while in the present embodiment, multiplesequences, for example two sequences, of adjustment factors are assignedto various possible base station cooperations, wherein one sequencecorresponds to the macro heavily loaded, and another one corresponds tothe macro lightly loaded. The two sequences may have some common part,and thus in step 22, possibly only the first sequence and the differentparts of the second sequence will be transmitted to UEs. In step 24, theUE adjusts the original CQIs based on said two sequences of adjustmentfactors respectively. Then, in step 25, the UE reports one highest CQIwith respect to each sequence. According to the present embodiment, thebase station can freely select the reported CQIs corresponding todifferent macro loads to perform traffic offloading.

Fourth Embodiment

In the above description, in step 21, each possible base stationcooperation is assigned with an adjustment factor according to loadconditions of the base station, and all the adjustment factors aretransmitted to the UEs through for example RRC or MAC or PDCCH. However,in case that the number of the possible base station cooperation islarge, the overhead for transmitting the adjustment factors becomes aproblem. Considering the overhead problem, in the present embodiment,instead of assigning an adjustment factor to each possible base stationcooperation, an adjustment factor is assigned to several possible basestation cooperations. For example, if a same adjustment factor isassigned to each two possible base station cooperations, the adjustmentfactors need to be transmitted to the UEs may reduce to half.Accordingly, the overhead can be reduced.

Fifth Embodiment

The above fourth embodiment proposes an improved technical solution forreducing the overhead. In the present embodiment, another improvedtechnical solution for reducing the overhead will be given.

In the present embodiment, in step 21, instead of assigning adjustmentfactors to various possible base station cooperations, each base stationis assigned with an adjustment factor. For example, the macro may beassigned a CQI reduction value of 6, and the LPN1 and LPN2 may beassigned CQI reduction values of 3 and 1, respectively. Then, in step24, the original CQIs calculated with respect to various possible basestation cooperations will be adjusted by subtracting corresponding CQIreduction values based on if the base stations transmit data in the basestation cooperation. For example, as for the original CQI calculatedbased on the base station cooperation that only the macro transmits datatherein, the CQI reduction value of 6 will be subtracted from theoriginal CQI; as for the original CQI calculated based on the basestation cooperation that all the macro and the LPNs transmit datatherein, all the CQI reduction values of 6, 3 and 1 will be subtractedfrom the original CQI.

The present embodiment enables very low overhead. For example, as for 3base stations cooperation, only 3 adjustment factors need to betransmitted to the UE instead of 19 adjustment factors as described inthe first embodiment.

Sixth Embodiment

The above embodiments are given by taking a macro and two LPNs (LPN1 andLPN2) carrying out the base station cooperation as an example. In fact,the method described above can be applied to the case of two basestations (such as a macro and a LPN) or more than three base stations(such as a macro and a plurality of LPNs).

As for the two base stations cooperation, there are 5 possible basestation cooperations and accordingly 5 CQI calculations hypotheses asshown in table 2.

TABLE 2 hypotheses Macro LPN 1 2 base stations data data 2 1 basestation data mute 3 data interference 4 mute data 5 interference data

The method of dynamically determining CQIs to be reported by UE in caseof two base stations cooperation is the same as that described in thefirst embodiment, which will not be repeatedly described.

In the above description, the method of dynamically determining CQIs tobe reported by UE according to the present application has beendisclosed. In the following, the structure of the base stationdetermining CQIs to be reported will be described with reference to FIG.3.

As shown in FIG. 3, the base station comprises assignment unit 31, whichassigns adjustment factors used to adjust original CQIs to variouspossible base station cooperations; an adjustment factor transmittingunit 32, which transmits the assigned adjustment factors to UserEquipments; a receiving unit 33, which receives the knowledge of whichCQI is reported and the corresponding CQI from User Equipments; and ascheduling unit 34, which allocates spectral resources to UserEquipments based on the knowledge of which CQI is reported and thecorresponding CQI.

It should be noted that instead of assigning adjustment factors tovarious possible base station cooperations, the assignment unit 31 canassign adjustment factors to base stations, like the above fifthembodiment of the present application. And the adjustment factors may bedecided based on the load conditions at multiple base stations or anyother factors such as the backhaul condition of network.

It should also be noted that the base station may further comprises areference signal and data transmitting unit, which transmits to UEs areference signal and relevant data which can be used for calculatingoriginal CQIs at UE.

In the following, the structure of the UE will be described withreference to FIG. 4. As shown in FIG. 4, the UE comprises an adjustmentfactor receiving unit 41, which receives adjustment factors from a basestation; a calculation unit 42, which calculates an original CQI foreach possible base station cooperation; an adjustment unit 43, whichadjusts the original CQIs based on the received adjustment factors; anda reporting unit 44, which reports to the base station one or multipleoriginal CQI(s) that are the highest after the aforesaid adjustment andknowledge of which CQIs are reported. It can be easily understood thatthe received adjustment factors are assigned for base stations orassigned for various possible base station cooperations by the basestation.

The calculation unit 42 can calculate the original CQIs in any wellknown manner. In a possible manner, the UE further comprises a referencesignal and data receiving unit 45, which receives from the base stationa reference signal and relevant data which can be used for calculatingoriginal CQIs, and the calculation unit 42 calculates the original CQIsbased on the received reference signal and relevant data.

The foregoing detailed description has set forth various embodiments ofthe devices and/or processes via the use of block diagrams, flowcharts,and/or examples. Insofar as such block diagrams, flowcharts, and/orexamples contain one or more functions and/or operations, it will beunderstood by those skilled in the art that each function and/oroperation within such block diagrams, flowcharts, or examples can beimplemented, individually and/or collectively, by a wide range ofhardware, software, firmware, or virtually any combination thereof. Inone embodiment, several portions of the subject matter described hereinmay be implemented via Application Specific Integrated Circuits (ASICs),Field Programmable Gate Arrays (FPGAs), digital signal processors(DSPs), or other integrated formats. However, those skilled in the artwill recognize that some aspects of the embodiments disclosed herein, inwhole or in part, can be equivalently implemented in integratedcircuits, as one or more computer programs running on one or morecomputers (e.g., as one or more programs running on one or more computersystems), as one or more programs running on one or more processors(e.g., as one or more programs running on one or more microprocessors),as firmware, or as virtually any combination thereof, and that designingthe circuitry and/or writing the code for the software and or firmwarewould be well within the skill of one of those skilled in the art inlight of this disclosure. In addition, those skilled in the art willappreciate that the mechanisms of the subject matter described hereinare capable of being distributed as a program product in a variety offorms, and that an illustrative embodiment of the subject matterdescribed herein applies regardless of the particular type of signalbearing medium used to actually carry out the distribution. Examples ofa signal bearing medium include, but are not limited to, the following:a recordable type medium such as a floppy disk, a hard disk drive, aCompact Disc (CD), a Digital Video Disk (DVD), a digital tape, acomputer memory, etc.; and a transmission type medium such as a digitaland/or an analog communication medium (e.g., a fiber optic cable, awaveguide, a wired communications link, a wireless communication link,etc.).

With respect to the use of substantially any plural and/or singularterms herein, those having skills in the art can translate from theplural to the singular and/or from the singular to the plural as isappropriate to the context and/or application. The varioussingular/plural permutations may be expressly set forth herein for sakeof clarity.

While various aspects and embodiments have been disclosed herein, otheraspects and embodiments will be apparent to those skilled in the art.The various aspects and embodiments disclosed herein are for purposes ofillustration and are not intended to be limiting, with the true scopeand spirit being indicated by the following claims.

What is claimed is:
 1. A method performed by a base station for receiving a channel quality indicator (CQI), the method comprising: setting a CQI adjustment value for a base station coordination type, the base station coordination type defining which base stations among a plurality of base stations perform coordinated transmission, the CQI adjustment value being set according to traffic loading conditions of the plurality of base stations; transmitting, to a user equipment (UE), the CQI adjustment value; receiving the CQI reported from the UE, the CQI being calculated at the UE in reference to the CQI adjustment value; and assigning a resource to the UE in reference to the received CQI.
 2. The method according to claim 1, wherein the base station coordination type is one of a plurality of base station coordination types that are hypotheses defining different coordinated transmissions performed by the plurality of base stations, each of which performs transmission of data or no transmission of data.
 3. The method according to claim 1, wherein the CQI adjustment value is transmitted to the UE through Radio Resource Control, Media Access Control or Physical Downlink Control Channel.
 4. The method according to claim 1, wherein the base station coordination type includes a high power node and one or a plurality of lower power nodes.
 5. The method according to claim 1, wherein the CQI is calculated at the UE by subtracting the CQI adjustment value from an original CQI.
 6. The method according to claim 1, wherein the CQI is calculated at the UE by multiplying an original CQI by the CQI adjustment value.
 7. The method according to claim 1, further comprising receiving, from the UE, information identifying the base station coordination type, for which the CQI is reported from the UE.
 8. A base station comprising: circuitry which, in operation, sets a channel quality indicator (CQI) adjustment value for a base station coordination type, the base station coordination type defining which base stations among a plurality of base stations perform coordinated transmission, the CQI adjustment value being set according to traffic loading conditions of the plurality of base stations; a transmitter which, in operation, transmits, to a user equipment (UE), the CQI adjustment value; and a receiver which, in operation, receives a CQI reported from the UE, the CQI being calculated at the UE in reference to the CQI adjustment value; and a scheduler which, in operation, assigns a resource to the UE in reference to the received CQI.
 9. The base station according to claim 8, wherein the base station coordination type is one of a plurality of base station coordination types that are hypotheses defining different coordinated transmissions performed by the plurality of base stations, each of which performs transmission of data or no transmission of data.
 10. The base station according to claim 8, wherein the transmitter transmits the CQI adjustment value to the UE through Radio Resource Control, Media Access Control or Physical Downlink Control Channel.
 11. The base station according to claim 8, wherein the base station coordination type includes a high power node and one or a plurality of lower power nodes.
 12. The base station according to claim 8, wherein the CQI is calculated at the UE by subtracting the CQI adjustment value from an original CQI.
 13. The base station according to claim 8, wherein the CQI is calculated at the UE by multiplying an original CQI by the CQI adjustment value.
 14. The base station according to claim 8, wherein the receiver receives, from the UE, information identifying the base station coordination type, for which the CQI is reported from the UE. 